1. Field of the Invention
This invention relates to solderable conductor patterns made of noble metal compositions which are adherent to substrates.
2. Description of the Prior Art
Conductor patterns made of compositions, or materials, which are applied to and fired on ceramic, or on ceramic substrates provided with a thick film dielectric layer usually comprise finely divided noble metal particles and inorganic binder particles. Such compositions are commonly applied to substrates as a "thick film," a dispersion of the inorganic materials of the compositions in an inert liquid medium or vehicle. The metallic component of the composition provides an electrical conductor having low electrical resistance while the inorganic binders, such as glass, bonds the metal particles to the substrate.
The most relevant prior art known to applicants is that found in U.S. Pat. No. 4,004,057 which issued on Jan. 18, 1977. This patent discloses patterns made of gold conductor compositions which patterns are formed on substrates by conventional thick film techniques which include firing the substrate after the pattern is printed on it, to produce patterns and pads of such patterns to which electronic components and their leads can be bonded using conventional thermal compression bonding techniques. The particular gold conductor compositions within the scope of the disclosure of U.S. Pat. No. 4,004,057 have excellent adherence to ceramic substrates and such substrates coated with thick film dielectric layers composed, for example, of mixtures of alumina and glass. However, the strength of the solder bond or joint between a pad made of the compositions, a lead pad, and a copper lead precoated with solder varied from being unacceptably weak to being acceptably strong.
The major problem with the prior art conductor composition is that the strength of solder joints between lead pads which are made from some compositions within the ranges disclosed in U.S. Pat. No. 4,004,057 and copper electronic leads soldered to the pad with a 50% lead 50% indium solder have widely varying strengths with some being unacceptably weak both initially and after being aged at elevated temperatures.
It is therefore an object of this invention to provide a solderable conductor pattern on a substrate in which the conductor composition adheres to fired dielectric surfaces of ceramic substrates and to the surfaces of such substrates without dielectric surfaces.
It is another object of this invention to provide an improved solderable conductor pattern on a substrate in which the conductor composition does not need to be physically or chemically cleaned prior to soldering beyond the action of noncorrosive fluxes compatible with the reliability requirements of electronic systems such as digital computers.
It is still another object of this invention to produce an improved solderable conductor pattern in which the gold conductor composition is optimized to maximize the strength of solder bonds between leads of electronic components and the portions, or lead pads of the pattern to which they are soldered while retaining the characteristic that the pattern does not need to be physically or chemically cleaned prior to soldering.